1. Field of the Invention
The present invention relates to a combustible gas detector for detecting a combustible gas, a process for producing the combustible gas detector, and a fuel cell system equipped with the combustible gas detector.
2. Description of the Related Art
As the development of hydrogen utilization technology such as fuel cells advances, the importance of combustible gas sensors increases so as to more safely control these systems.
For example, it is known that hydrogen explodes if it is present at a concentration of 4 to 75% in the air. If a combustible gas leaks to the outside due to a system failure or when a purging (scavenging) operation is necessary for driving the system, it is necessary to keep an ignition source away from the explosible concentration area. Recently, there has been a demand not only for the development of in-car and at-home stationary types of fuel cells, but also for the development of small-size fuel cells for mobile devices as a substitute for conventional secondary batteries advances. Furthermore, there is a demand for miniaturization and low power consumption of combustible gas sensors.
Conventionally, various types of combustible gas sensors have been developed and marketed. These are roughly classified into three types, that is, an adsorption type, a catalytic combustion type, and a gas heat conduction type. These sensors are used respectively depending on a gaseous species, a detection region, or response speed. Of these, the adsorption-type combustible gas sensor measures the change of electric resistance or volume expansion when a gas is adsorbed onto the surface of a metal oxide semiconductor (tin oxide) or the like. The catalytic combustion-type combustible gas sensor measures the change in resistance associated with the increase in temperature caused by catalytic combustion of a gas at the surface of a catalyst (platinum line or the like). The gas heat conduction-type combustible gas sensor measures the change in the electric resistance associated with the change in the temperature of a heating element (platinum coil or the like) by the difference in the specific thermal conductivity of a gas.
The development adopting semiconductor processing technique has been hitherto advanced to achieve miniaturization and low power consumption of these combustible gas sensors. The semiconductor processing technique is a fine processing technique for manufacturing LSI using a material such as a silicon wafer. A so-called MEMS (micro-electro-mechanical systems) technique has been extensively developed and already put into practical use in the production of acceleration sensors for automobiles.
D. R. Baselt et al., Sensors and Actuators B88 (2003) 120-131 provides an example of miniaturizing a combustible gas sensor using a semiconductor processing technique. This sensor is an adsorption type sensor in which a palladium film is formed on a cantilever made of Si. In this sensor, when a combustible gas is adsorbed to the palladium film, the film expands and deflects the cantilever. This deflection is detected by the change of the capacitance between the electrode on the cantilever and the electrode on the substrate.
As a catalytic combustion-type combustible gas sensor, sensors provided with a catalyst material and a thermoelectric conversion element at the same time are proposed as shown, for example, in Japanese Patent Application Laid-Open No. 2003-156461. In this sensor, heat generated by the catalytic reaction between a combustible gas and the catalyst material is converted into a voltage signal by a thermoelectric conversion effect, and this is used as a detection signal. Furthermore, Japanese Patent Application Laid-Open No. 2005-300522 discloses technology for further miniaturizing such a sensor using a semiconductor processing technique.
When hydrogen and oxygen come into contact with each other in the presence of a catalyst, catalytic combustion occurs by the following reaction and water is generated.
            H      2        +                  1        2            ⁢              O        2              ->            H      2        ⁢    O  
The Gibbs free energy change at 25° C. in this reaction is −237.2 kJ/mol, and this is assumed as the heat of catalytic combustion.
Shape-memory alloys and bimetals are conventionally well-known as materials, which transform with temperature. The shape-memory alloys, such as a TiNi alloy, transform into a shape preliminarily memorized at a predetermined temperature or more. The bimetals include two pieces of metal plates with different coefficients of thermal expansion.
Utilizing the property of a change in the bending depending on a change in temperature, the bimetals are used for thermometers and temperature control units. Examples of bimetals using fine processing are disclosed in Japanese Patent Application Laid-Open No. 2000-246676, and an example of forming a relay is provided in H. Jerman, J. Micromech. Microeng. 4 (1994) 210-216.
The relationship between the temperature and the distortion in the cantilever structure of a bimetal material is shown as follows by S. Timoshenko, J. Opt. Soc. Am., 11 (1925) 233-255.
At first, curvature ρ and displacement y of the tip when the temperature of a bimetal including the first layer having a thickness t1 and a length l and the second layer having a thickness t2 and a length l is increased by ΔT are represented by the following expression:
            1      ρ        =                  6        ⁢                  (                                    α              1                        -                          α              2                                )                ⁢        Δ        ⁢                                  ⁢                  T          ⁡                      (                                          t                1                            +                              t                2                                      )                          ⁢                  t          1                ⁢                  t          2                ⁢                  E          1                ⁢                  E          2                                      3          ⁢                                    (                                                t                  1                                +                                  t                  2                                            )                        2                    ⁢                      t            1                    ⁢                      t            2                    ⁢                      E            1                    ⁢                      E            2                          +                              (                                                            t                  1                                ⁢                                  E                  1                                            +                                                t                  2                                ⁢                                  E                  2                                                      )                    ⁢                      (                                                            t                  1                  3                                ⁢                                  E                  1                                            +                                                t                  2                  3                                ⁢                                  E                  2                                                      )                                    y    =          2      ⁢      ρ      ⁢                          ⁢                                    sin            2                    ⁡                      (                          l                              2                ⁢                ρ                                      )                          .            
In the above expression, E1 and E2 are longitudinal elastic coefficients of the respective layers, and α1 and α2 are thermal expansion coefficients of the respective layers.
Here, representative material values of various materials are shown in Table 1. By adequately selecting materials of the first layer and the second layer, bimetals with which a desired distortion can be obtained when a difference in the temperature occurs.
TABLE 1Young'sLinear expansionmodulusPoissoncoefficient[GPa]ratio[×10−6/K]Stainless1970.310-17Nickel199.50.3113.3Silicon1600.287.6Aluminum72.60.34523.5Platinum1700.388.8Gold78.50.2714.1Copper129.80.3417Pyrex630.22.8Quartz730.170.55Polyimide345-56Polyethylene0.4-1.30.458100-200
Likewise, the relationship between temperature and distortion when a bimetal material is formed not on a cantilever, but on a diaphragm, is shown in the above-mentioned H. Jerman, J. Micromech. Microeng. 4 (1994) 210-216 as follows. When the internal radius of the diaphragm (part not displaced) is assumed as b and the external radius as a:
  y  =                    K        y                    (                  1          +          v                )              ⁢    Θ    ⁢                  ⁢                  a        2            .      
In the above expression, ν is the Poisson Ratio of the diaphragm and Ky is a constant determined by the boundary condition.
In addition:
      Θ    =                  6        ⁢                  (                                    α              1                        -                          α              2                                )                ⁢        Δ        ⁢                                  ⁢                  T          ⁡                      (                                          t                1                            +                              t                2                                      )                          ⁢                  (                      1            +            v                    )                                      t          2          2                ⁢                  K          1                                K      1        =          4      +              6        ⁢                              t            1                                t            2                              +              4        ⁢                              (                                          t                1                                            t                2                                      )                    2                    +                                    E            1                    ⁢                                    t              1              3                        ⁡                          (                              1                -                                  v                  2                                            )                                                            E            2                    ⁢                                    t              2              3                        ⁡                          (                              1                -                                  v                  1                                            )                                          +                                                  E              1                        ⁢                                          t                1                            ⁡                              (                                  1                  -                                      v                    2                                                  )                                                                        E              2                        ⁢                                          t                2                            ⁡                              (                                  1                  -                                      v                    1                                                  )                                                    .            
Small fuel cells have attracted attention as an energy source to be mounted on small electric apparatuses. The reason why the fuel cell is useful as a drive source of small electric apparatuses is that the amount of energy that can be supplied per volume and per weight thereof is several times to nearly 10 times greater than that of conventional secondary lithium ion batteries. In particular, hydrogen is the most suitable fuel in order to obtain a large output from a fuel cell. However, hydrogen is a gas at normal temperature, and it is necessary to store hydrogen in a small fuel tank at a high density.
The following methods for storing hydrogen in this manner are known.
The first method is compressing and storing hydrogen as a high-pressure gas. The hydrogen density per volume is about 18 mg/cm3 when the gas in the tank is compressed to 200 atm.
The second method is a method to cool hydrogen to a low temperature and store it as liquid. Liquefying hydrogen requires a large amount of energy and is also problematic, because liquid hydrogen may spontaneously vaporize and leak. However, this method enables high-density storage.
The third method is storing hydrogen using a hydrogen-storing alloy. A problem with this method is that due to a large specific gravity of the hydrogen storing alloys, hydrogen at merely 2 wt % or so can be occluded and the fuel tank is heavy. However, because the volume of the occluded hydrogen is large, this method is effective for miniaturization.
Power generation by a polymer electrolyte fuel cell is performed as follows. Perfluorosulfonic acid-based cation-exchange resin is often used for a polymer electrolyte membrane.
For example, Nafion by DUPONT is well known for such a membrane.
A membrane electrode assembly, which is formed by sandwiching a polymer electrolyte membrane with a pair of porous electrodes carrying a catalyst such as platinum, that is, with a fuel electrode and an oxidizer electrode provides a power generation cell.
When an oxidizer is supplied to the oxidizer electrode while a fuel is supplied to the fuel electrode in this power generation cell, protons move in the polymer electrolyte membrane, and power is generated.
Japanese Patent Application Laid-Open No. H8-315847, for example, proposes the following safety measure in case fuel leaks from a fuel cell.
This measure includes providing a blocking unit, which blocks the supply of the fuel to the main body of the fuel cell when fuel leaks out, and linking this blocking procedure to the detection procedure by a fuel detection sensor.
As for the fuel detection unit of the fuel cell, methods for detection via a decrease in the power generation of the fuel cell, or, when the fuel is a gas, methods by mixing detection materials, such as odor-emitting materials or helium, and the like methods have been tried.
Also, solenoid valves have been used as a unit for blocking the fuel.
In small fuel cells, a method of supplying a consumed amount of fuel from a fuel tank with the outlet in a closed condition and without circulating the fuel (dead end method) is often used. However, there is a problem in that impurity gases, such as nitrogen and steam, penetrate the electrolyte film and are accumulated in the fuel channel, which causes a decrease in the power generation over time. Therefore, a scavenging (purging) procedure is often performed to drain the accumulated impurity gases in the fuel cells adopting the dead-end method. The ON/OFF change of the purging procedure was controlled by time, the output of the fuel cell, and gas concentration in the fuel channel.
However, conventional gas sensors for detecting combustible gas described above have the following problems.
For example, the adsorption-type gas sensors have very poor stability and response at the normal temperature, and a method of warming the device with a heater is used to improve the response. However, when a heater is used, electricity should be continuously applied to the exothermic body even during standby time, which causes an increase in the power consumption of the sensor. The gas heat conduction-type combustible gas sensors are not suitable for detecting combustible gas at a concentration of about several percent, because the combustible gas concentration (hereinbelow, referred to as detection limit concentration) detectable by such sensors is high. In addition, electricity should be continuously applied to the exothermic body even during standby time, which increases power consumption of the sensor. Besides, conventional adsorption-type, catalytic combustion-type, gas heat conduction-type and the like combustible gas sensors detect a change in the voltage or electrical resistance as an analog signal and, consequently, are susceptible to noise. Furthermore, miniaturization of the processing circuit of the detection signal is difficult, and many of the sensors always consume electricity.
Because conventional combustible gas detectors as described above have problems, such as insufficient miniaturization and increased power consumption, they require a system having a larger size and excess energy in the case of using fuel cells for small size electric apparatuses.